Backlight using planar hologram for flat display device

ABSTRACT

The backlight for a flat display device including a light source, a light guide plate and a surface hologram is provided. The light guide plate is installed at one side of the light source, and light from the light source travels in the light guide plate while being totally reflected. The surface hologram is formed on at least one surface of the light guide plate. The surface hologram has a pattern of a predetermined grating interval and a predetermined grating depth in order to diffract light at a predetermined angle toward the light guide plate.

BACKGROUND OF THE INVENTION

This application claims the priorities of Korean Patent Application Nos.2001-38807 and 2002-23114, filed on Jun. 30, 2001 and Apr. 26, 2002,respectively, in the Korean Intellectual Property Office, thedisclosures of which are incorporated herein in their entireties byreference.

FIELD OF THE INVENTION

The present invention relates to the field of backlights for flatdisplay devices, and more particularly, to a backlight for a flatdisplay device including a light path changing unit, which makes alreadyincident light again incident upon a planar hologram at an angleproviding the maximum diffraction efficiency, and a light guide plateincluding the planar hologram.

DESCRIPTION OF THE RELATED ART

Referring to FIG. 1, a conventional backlight includes a light source11, a light guide plate 13, a diffusion plate 19 and prismatic plates 21a and 21 b. The light guide plate 13 directs light emitted from thelight source 11, the diffusion plate 19 diffuses light directed by thelight guide plate 13, and the prismatic plates 21 a and 21 b collectdiffused light on their respective front sides.

Referring to FIG. 2, in a conventional backlight, light emitted from thelight source 11 is incident upon the light guide plate 13 and furthertransmitted by total reflection. Some of the light passes through thebottom surface of the light guide plate 13, is reflected by a reflectionplate 17 and reenters the light guide plate 13.

A light scattering pattern 23, which is a dot pattern, is printed on thebottom surface of the light guide plate 13. From light incident upon thelight guide plate 13, only light not reflected due to scattering by thelight scattering pattern 23 penetrates through the upper surface of thelight guide plate 13.

To improve light uniformity, the diffusion plate 19, which covers theentire surface of the light guide plate 13, diffuses the light emittedfrom the light guide plate 13.

The prismatic plates 21 a and 21 b are provided on the entire surface ofthe diffusion plate 19. Each of the prismatic plates 21 a and 21 b isformed of a plurality of prism strips with triangular cross sections.When two prismatic plates are used together, as in the case of theprismatic plates 21 a and 21 b, they are disposed so that the prismaticstrips of the prismatic plate 21 a make a right angle with the prismaticstrips of the prismatic plate 21 b. The prismatic plates 21 a and 21 bincrease the front luminance of light by refracting or totallyreflecting light emitted from the diffusion plate 19 depending on theangle at which light is incident upon the prismatic plates 21 a and 21b.

A protection plate 25 is installed on the front side of the prismaticplate 21 a to protect the entire system of backlight.

In such a conventional backlight, in order to emit light from the lightguide plate 13, a dot pattern for light scattering is formed on thebottom surface of the light guide plate 13, or the light guide plate 13has a prismatic or sinusoidal bottom surface. However, manufacturing ofa prismatic or sinusoidal pattern on the light guide plate 13 requires aspecial and expensive equipment and a time-consuming process.

The dot pattern printed on the bottom surface of the light guide plate13 makes spots appear on the screen when it is combined with a liquidcrystal panel, thus degrading image quality. A diffusion plate adoptedto solve this problem reduces the light reflection efficiency by 20% to30% depending on the transmission performance. As the light efficiencyof the light guide plate 13 depends on the position and area of its dotpattern, it may be further reduced according to the position and area ofthe dot pattern.

In a conventional backlight, light emitted from the light source 11 isscattered by a dot pattern for light scattering or prism-shaped groovesformed on the bottom surface of the light guide plate 13, and thenemitted at an angle of 70° to 90° with respect to a normal lineperpendicular to the light guide plate 13. A diffusion plate and aprismatic plate are further required to convert the direction of suchlight emitted at a large angle into the normal direction of the lightguide plate 13. As a result, the assembling process of a backlight iscomplicated, and the manufacturing costs thereof increase.

FIGS. 3A through 3E are graphs showing a variation in the distributionof light intensity for a conventional backlight. FIG. 3A shows the lightintensity distribution of light emitted from the light guide plate 13 ofFIG. 2. Referring to FIG. 3A, the light intensity distribution of lightemitted from the light guide plate 13 has an asymmetrical structurewhere the highest light intensity is obtained around 90°. That is, thelight intensity around 90° is about 3,400 cd (where cd denotes the baseunit of light intensity, i.e., candela), and a light intensity of 400 cdor less appears between 0° and 180°.

The asymmetrical light intensity distribution of FIG. 3A changes into asymmetrical light intensity distribution in which the light intensity ofthe center portion of a conventional backlight becomes stronger, asshown in FIGS. 3B to 3E.

FIG. 3B shows the light intensity distribution of light emitted from thelight guide plate 13 and the diffusion plate 19. Referring to FIG. 3B,an asymmetrical distribution is still shown where a portion with thehighest light intensity of about 800 cd appears on the upper halfvertical axis and the light intensity becomes weaker from the center ofthe conventional backlight toward the periphery thereof.

FIG. 3C shows the light intensity distribution of light transmitted bythe light guide plate 13, the diffusion plate 19 and the first prismaticplate 21 b. Referring to FIG. 3C, the highest light intensity of about940 cd appears around the center of the conventional backlight. Thelight intensity distribution of FIG. 3C is symmetrical in contrast withthe light intensity distribution of FIG. 3B.

FIG. 3D shows the light intensity distribution of light transmitted bythe light guide plate 13, the diffusion plate 19 and the first andsecond prismatic plates 21 b and 21 a. The light intensity distributionof FIG. 3D has a similar shape to that of FIG. 3C rotated by 90°. Thelight intensity at the center of the conventional backlight is about1,220 cd, and the distribution of light intensity is symmetrical.

FIG. 3E shows the distribution of the final light intensity of theconventional backlight. Referring to FIG. 3E, the light intensity at thecenter of the conventional backlight is about 1,100 cd, and thedistribution of the final light intensity is symmetrical.

Conventional backlights must include a prismatic plate in order toobtain such a symmetrical light intensity distribution as shown in FIG.3E. This leads to complicated, expensive backlights.

As described above, in the prior art, an additional device is requiredto compensate for the large emission angle of light emitted from a lightguide plate. This causes an increase in the manufacturing costs ofbacklights.

SUMMARY OF THE INVENTION

To solve the above-described problems, it is an object of the presentinvention to provide a backlight for a flat display device capable ofreducing light loss, which is generated while light is passing through adiffusion plate and a prismatic plate so that light is output from alight guide plate at an angle of 90° or at a predetermined angle near90° with respect to the plane of the light guide plate, or to provide ano plastic plate backlight for a flat display device.

Another object of the present invention is to provide a backlight for aflat display device capable of using specific polarized light dependingon the specific wavelength of light emitted from a light source.

Still another object of the present invention is to provide a backlightfor a flat display device capable of compensating for the inverseproportional relationship between the diffraction efficiency and lightintensity of a light guide plate on which a diffraction grating patternis formed, both the diffraction efficiency and the light intensitydepending on the incidence angle.

In order to achieve the above objects of the present invention, there isprovided a backlight for a flat display device including a light source,a light guide plate and a surface hologram. The light guide plate isinstalled at one side of the light source. In the light guide plate,light from the light source travels while being totally reflected. Thesurface hologram is formed on at least one surface of the light guideplate. The surface hologram diffracts and emits the light at apredetermined angle to the plane of the light guide plate.

Preferably, the backlight for a flat display device further includes alight path changing unit installed at one side of the light guide plate.The light path changing unit changes the path of light traveling in thelight guide plate to make light incident upon the planar hologram at anangle near the angle providing the maximum diffraction efficiency.

The backlight for a flat display device can further include a reflectingplate installed on the rear side of the light guide plate. Thereflecting plate reflects light diffracted by the planar hologram andsends the diffracted light back to the light guide plate.

The light path changing unit is a reflective mirror that is locatedopposite to the light source and inclined at a predetermined angle.Alternatively, the light path changing unit is a reflective surface ofthe light guide plate, the reflective surface being located opposite tothe light source and inclined at a predetermined angle. Stillalternatively, the light path changing unit is a refracting elementinstalled between the light source and the light guide plate orinstalled opposite to the side of the light guide plate where the lightsource is installed.

Here, the refracting element is either a refractive lens or a refractivegrating.

Preferably, the diffraction efficiency of the planar hologram becomeslower toward either the light source or the light path changing unit.

Preferably, the pattern of the planar hologram becomes smaller towardeither the light source or the light path changing unit.

It is preferable that the grating interval of the planar hologram is 2μm or less.

In order to diffract incident beams into optimal states, the gratinginterval of the planar hologram can be composed of at least two types ofgrating intervals depending on the wavelength of light.

As for the surface hologram, the grating depth can be set so that thepolarization of light output from the light guide plate after beingdiffracted by the surface hologram is superior to the direction ofspecific polarized light. Accordingly, it is preferable that the gratingdepth of the surface hologram is set so that the diffraction efficiencyof a specific polarized light beam is 1.5 times as large as or greaterthan that of the other polarized light that meets the specific polarizedlight beam at a right angle.

Preferably, the backlight for a flat display device further includes adiffusion plate installed on the entire surface of the light guide plateto diffuse light emitted from the light guide plate.

When a white light source is used, the diffusion can reduce the degreeof color separation of light caused by the difference in diffractionangle between light wavelengths.

In the present invention, a light path changing unit can increase theoutput amount of light by changing the path of high intensity lightincident at an angle near 90°, so that the incident light is againincident upon the surface hologram at an angle guaranteeing the maximumdiffraction efficiency.

Also, the planar hologram formed on the light guide plate enables lightincident upon the light guide plate to be output from the light guideplate at an angle nearly perpendicular to the light guide plate.Accordingly, an efficient backlight having a simple structure having noprismatic plates can be provided.

In addition, the diffraction efficiency of a specific polarized lightbeam is rendered greatly higher than that of other polarized light beamsby controlling the grating interval and depth of the surface hologram.Thus, the specific polarized light beam of light emitted from the lightguide plate becomes excellent. Finally, the backlight according to thepresent invention can increase the light efficiency using polarization.As a result, a backlight using specific polarization can bemanufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a schematic perspective view of a conventional backlight;

FIG. 2 is a cross-section of a conventional backlight;

FIG. 3A shows the light intensity distribution of light transmitted by alight guide plate in a conventional backlight;

FIG. 3B shows the light intensity distribution of light transmitted by adiffusion plate in a conventional backlight;

FIG. 3C shows the light intensity distribution of light transmitted by afirst prismatic plate in a conventional backlight;

FIG. 3D shows the light intensity distribution of light transmitted by asecond prismatic plate in a conventional backlight;

FIG. 3E shows the light intensity distribution of final light in aconventional backlight;

FIG. 4A is a schematic cross-section of a backlight according to a firstembodiment of the present invention;

FIG. 4B is a schematic cross-section of a backlight according to asecond embodiment of the present invention;

FIG. 5A is a schematic cross-section of a backlight according to a thirdembodiment of the present invention;

FIG. 5B is a schematic cross-section of a backlight according to afourth embodiment of the present invention;

FIG. 6A is a schematic cross-section of a backlight according to a fifthembodiment of the present invention;

FIG. 6B is a schematic cross-section of a backlight according to a sixthembodiment of the present invention;

FIG. 7 is a schematic cross-section of a flat backlight having no lightpath changing units, according to an embodiment of the presentinvention;

FIG. 8 is an enlarged cross-section of a portion indicated by referencecharacter A of FIG. 7;

FIG. 9 is a graph showing the diffraction efficiency, light intensitydistribution and output light quantity with respect to incidence anglesof the flat backlight of FIG. 7;

FIG. 10A shows a stripe-patterned planar hologram formed on a lightguide plate in the backlight of FIG. 7;

FIG. 10B shows a stripe-patterned planar hologram formed on a lightguide plate in the backlight of FIG. 4A;

FIG. 11A shows a rectangle-patterned planar hologram formed on a lightguide plate in the backlight of FIG. 7;

FIG. 11B shows a rectangle-patterned planar hologram formed on a lightguide plate in the backlight of FIG. 4A;

FIG. 12A shows a stripe-patterned planar hologram formed on a lightguide plate in the backlight of FIG. 7;

FIG. 12B shows a stripe-patterned planar hologram formed on a lightguide plate in the backlight of FIG. 4A;

FIG. 13 schematically shows a method of forming a flat hologram used bya backlight for a flat display device according to the presentinvention;

FIG. 14 is a graph showing the diffraction efficiency of P-polarizedlight and S-polarized light according to the depths of the grating of aplanar hologram when light with a 460 nm wavelength and light with a 620nm wavelength are incident upon a planar hologram with a gratinginterval of 440 nm;

FIG. 15A shows the light intensity distribution of light transmitted bya light guide plate in the backlight of FIG. 7; and

FIG. 15B shows the light intensity distribution of final lighttransmitted by a diffusion plate in the backlight of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 4A through 6B show backlights for a flat display device accordingto the first through sixth embodiments of the present invention,respectively, the backlights having a light path changing unit. In FIGS.4A and 4B, the light path changing unit is a slant reflection planeformed on the side of a light guide plate.

Referring to FIG. 4A, a backlight for a flat display device according toa first embodiment of the present invention includes a light source 61,a light guide plate 63, a reflection plane 69 a, a planar hologram 65and a reflection plate 67. The light source 61 emits light l1. The lightguide plate 63 directs the light l1 emitted from the light source 61 bytotally reflecting the light l1. The reflection plane 69 a is used as alight path changing unit for changing the path of the light l1. Theplanar hologram 65 diffracts light to output light at an angle of noless than 80° or at a predetermined design angle with respect to theplane of the light guide plate 63. The reflection plate 67 is positionedat the bottom surface of the planar hologram 65, reflects lightdiffracted by the planar hologram 65, and sends reflected light back tothe light guide plate 63.

The light source 61 can be one of a laser diode (LD), a light emittingdevice (LED) and a cold cathode fluorescent lamp (CCFL). As describedabove, the planar hologram 65 is used in the present invention.

Generally, holograms can be classified into surface holograms and volumeholograms. In volume holograms, an image is recorded within a hologrammaterial. In surface holograms, an image is recorded on its surface.Consequently, a surface hologram can be mass-produced throughduplication, but usually provides low diffraction efficiency. However,when a hologram with a grating interval having a similar length to thewavelength of light is formed on a light guide plate, light is notscattered in many directions due to the small number of diffractionorders capable of being transmitted. This case can overcome lowdiffraction efficiency.

A backlight for a flat display device according to the present inventionreduces the, dependency upon the diffraction angle of reproduced lightusing a planar hologram and also compensates for the defects of theplanar hologram by increasing the diffraction efficiency by introducinga light path changing unit.

In the backlight for a flat display device of FIG. 4A, the light l1emitted from the light source 61 is incident upon the light guide plate63 at an angle where total reflection can be achieved with respect tothe plane of the light guide plate, and travels through totalreflections. Here, light intensity increases as an initial incidenceangle A approaches 90°, and the diffraction efficiency with respect tothe planar hologram 65 is highest at an incidence angle C rangingbetween 40° and 60°. Accordingly, the initial incidence angle A of lightis set at an angle between the total reflection angle Θ and 90°. Theincidence angle C is set to an angle where the refraction efficiencywith respect to the planar hologram 65 is the highest, by controllingthe slant angle of the reflection plane 69 a.

The total reflection angle can be obtained through the Snell's law.Since the refractive index (n) of the light guide plate 63 is usuallyabout 1.5, the total reflection angle Θ for totally reflecting the lightl1 within the light guide plate 63 can be obtained from Expression 1:

$\begin{matrix}{{a\;{\sin( \frac{1}{n} )}} - {a\;{\sin( \frac{1}{1.5} )}} - {41.8{^\circ}}} & (1)\end{matrix}$

When the incidence angle A of the light l1 ranges from the totalreflection angle Θ to 90° and the angle for the highest diffractionefficiency is 40°, the incidence angle C with respect to the planarhologram 65 is obtained from Equation 2:C=A−2B  (2)The inclination B of the reflection plane 69 a ranges from 0.9° to 25°according to Expression 3:

$\begin{matrix}{{0.9 - \frac{41.8 - 40}{2}} < {B - \frac{A - C}{2}} < {\frac{90 - 40}{2} - 25}} & (3)\end{matrix}$

Since the light amount increases as the incidence angle A approaches90°, in order to maximally diffract light having an incidence angle Aclose to 90°, for example, an 85° incidence angle A, a 17.5° inclinationangle B for a reflection plane can be obtained by substituting 85° forthe initial incidence angle A in Equation 2 and 50° for the incidenceangle C in Equation 2. The incidence angle of light upon the planarhologram 65, which provides the maximum diffraction efficiency, can beproperly controlled according to the pattern of the planar hologram 65because it depends on grating depths and grating intervals.

If light is incident upon the planar hologram 65 at an incidence angle Cclose to the incidence angle providing the maximum diffractionefficiency, light diffracted by the planar hologram 65 is reflected bythe reflection plate 67 and penetrates through the planar hologram 65.Some of the light transmitted by the planar hologram 65 re-enters thelight guide plate 63, and the rest is output through the light guideplate 63. Here, the output angle of light is set to an angle of 80° ormore by controlling the grating interval and the grating depth. Thiswill be described in detail when a backlight according anotherembodiment of the present invention having no light path changing unitsis described. In some cases, the angle at which light is output can becontrolled by adjusting the grating interval.

FIG. 4B shows a backlight according to a second embodiment of thepresent invention. Unlike the backlight for a flat display deviceaccording to the first embodiment of the present invention of FIG. 4A,the planar hologram 65 is formed on the upper side of the light guideplate 63. In addition, a reflection plane 69 b is inclined in theopposite direction to the reflection plane 69 a of FIG. 4A, so that theincidence angle C with respect to the plane of the planar hologram 65approaches the incidence angle providing the maximum diffractionefficiency. However, as long as the incidence angle C satisfies thetotal reflection angle, the reflection plane 69 b may not be inclined inthe opposite direction to the reflection plane 69 a of FIG. 4A. If ahologram pattern is formed on the upper surface of a light guide plate,like in this embodiment, a reflection sheet 67 may not be included.

As described above, a maximum output light amount is extracted by makinglight with high light intensity incident upon the planar hologram 65,which is formed on the upper surface of the light guide plate 63, at anangle close to the angle providing the maximum diffraction efficiency.

Backlights of FIGS. 5A and 5B adopt reflection mirrors 68 a and 68 b,respectively, instead of the reflection planes 69 a and 69 b of thebacklights according to the first and second embodiments of the presentinvention. FIG. 5A shows a backlight for a flat display device accordingto a third embodiment of the present invention.

Referring to FIG. 5A, light #3 is incident upon the upper side of thelight guide plate 63 at an incidence angle A. The incident light l3passes through the light guide plate 63 and is then reflected by thereflection mirror 68 a. The reflected light l3 re-enters into the lightguide plate 63 and is then incident upon the planar hologram 65 at anincidence angle C. As described above, when the incidence angle A isclose to 90°, the light intensity of the light l3 is the highest. Thelight l3 is incident upon the planar hologram 65 at the incidence angleC close to the angle providing the maximum diffraction efficiency.

FIG. 5B shows a backlight according to a fourth embodiment of thepresent invention. Referring to FIG. 5B, the light guide plate 63 hasthe planar hologram 65 formed on its upper side. In order to change theincidence angle of light l4 upon the planar hologram 65, the inclinationangle B of the reflection mirror 68 b is set in the same way as theinclination angle B of the reflection mirror 68 a for the backlight ofFIG. 5A.

FIG. 6A shows a backlight according to a fifth embodiment of the presentinvention. Referring to FIG. 6A, a diffraction grating 66 a is includedas a light path changing unit between the light source 61 and the lightguide plate 63. The diffraction grating 66 a changes the path of lightl5 incident upon the plane of the light guide plate 63 at an angle closeto 90°, so that the light l5 travels to be incident upon the planarhologram 65 at an angle close to the incidence angle providing themaximum diffraction efficiency. The incidence angle C with respect tothe planar hologram 65 has a range as described above, and the angle forthe maximum diffraction efficiency may vary within the range of theincidence angle C, depending on grating depths and grating intervals.Here, the diffraction grating 66 a can be replaced by a refraction lens.

FIG. 6B schematically shows a backlight for a flat display deviceaccording to a sixth embodiment of the present invention. Referring toFIG. 6B, a refraction lens 66 b is located opposite the light source 61,and has a reflective right side. Light l6 is reflected by the reflectiveright side and sent back to the light guide plate 63. The reflectedlight is incident upon the planar hologram 65 at an angle C close to theangle for the maximum diffraction efficiency. The range of the angle Cis the same as described above.

The backlights for a flat display device according to the first throughsixth embodiments of the present invention are just examples of thepresent invention. Other types of light path changing units can be usedif they can change the path of light and make light incident upon aplanar hologram at an angle close to the incidence angle for the maximumdiffraction efficiency. The new light path changing units used can beinstalled between the light source 61 and the light guide plate 63, bybeing formed on one side of the light guide plate 63, or by beinginstalled opposite the light source 61.

In addition, in the backlights for a flat display device according tothe first through sixth embodiments of the present invention, a planarhologram 65 with a pattern to be described later is formed on one sideof the light guide plate 63. That is, a planar hologram 65 with apattern providing low diffraction efficiency or a small pattern isformed at the position having a high light intensity, so that the outputamount of polarized light can be maximized and the output light can beuniformly distributed. The output light can be further uniformlydistributed by adding a diffusion plate over the light guide plate 63.

FIG. 7 schematically shows a flat backlight with no light path changingunits, according to the present invention. The flat backlight of FIG. 7includes a light guide plate 43, a light source 41, a reflecting plate47, and a planar hologram 45. The light source 41 is installed on oneside of the light guide plate 43, the reflecting plate 47 is installedunder the light guide plate 43, and the planar hologram 45 is formed onthe bottom surface of the light guide plate 43.

A diffusion plate 49 is further installed over the light guide plate 43.A protection plate (not shown) can be further installed over thediffusion plate 49. The planar hologram 45 can be also formed on theupper surface of the light guide plate 43.

Light 31, white light emitted from the light source 41 to the plasticlight guide plate 43, travels while being totally reflected within thelight guide plate 43. The light 31 continuously remains within the lightguide plate 43 as long as it satisfies the total reflection conditions.The light 31 is diffracted by the planar hologram 45 formed on thebottom surface of the light guide plate 43. First-order light beams 33,35 and 37, from the diffracted light beams, are reflected by thereflecting plate 47 and sent back to the light guide plate 43. Thereflected first-order light beams 33, 35 and 37 are output from thelight guide plate 43 at angles nearly perpendicular to the plane of theplastic guide 43, that is, at angles of 80° or more. Meanwhile, a zeroorder light beam 39 is again reflected by the planar hologram 45 becauseit satisfies the total reflection conditions.

Most light is incident at an angle of 80° or more because the lightguide plate 43 is long and narrow. Hence, when green light with awavelength (λ) of 540 nm is incident at an angle (θ) of 80°, first orderdiffracted light is emitted vertically by satisfying Expression 4:

$\begin{matrix}{P - \frac{\lambda}{n \times \sin\;\theta} - \frac{540}{1.5 \times \sin\mspace{20mu} 80{^\circ}} - {365\mspace{14mu}{nm}}} & (4)\end{matrix}$wherein P denotes the grating interval, which is formed on a planarhologram.

As shown in Equation 4, when green light is incident at 80° and thegrating interval P of the planar hologram 45 is about 365 nm, it isemitted nearly perpendicularly to the light guide plate. When light isincident at a different angle than 80° or light having a differentwavelength than green light is incident, the grating interval P of theplanar hologram 45 is changed to vertically emit the incident light. Inall of these cases, when the grating interval of the planar hologram 45is about 2 μm or less, light is uniformly emitted from the light guideplate 43 at a certain angle.

In the backlight of FIG. 7, light emitted from the light guide plate 43makes an angle of almost 90° to the plane of the light guide plate 43,that is, an angle of 80° or more. Accordingly, the backlight of FIG. 7does not require a prismatic plate, which is used in conventionalbacklights to obtain a high luminous efficiency. This simplifies thestructure of a backlight, so that the backlight can be easilymanufactured. Even when there is no need to emit light at an angle ofalmost 90°, light can be emitted at a certain angle by controlling thegrating interval.

FIG. 8 is an enlarged cross-section of a portion indicated by referencecharacter A of FIG. 7, showing the path of travel of incident light 31diffracted by the planar hologram 45. Referring to FIG. 8, zero orderlight 39 from the incident light 31 is reflected by the interface at thehologram 45 formed on the bottom surface of the reflecting plate 47,while first-order diffracted light beams 33, 35 and 37 are diffracted bythe hologram 45. That is, the first-order diffracted light beam 35,which is green, is diffracted at an angle of almost 90° to the plane ofthe light guide plate 43, while the first order light beams 33 and 37,which are red and blue, respectively, are diffracted at an angle largerthan the diffraction angle of the green light beam 35. First-orderdiffracted light beams 33, 35 and 37 are reflected by the reflectionplate 47 and sent back to the light guide plate 43 at an angle smallerthan the total reflection angle. The first-order diffracted light beams33, 35, and 37 not satisfying the total reflection conditions get out ofthe light guide plate 43 and travels at an angle of 80° or more to theplane of the light guide plate 43.

FIG. 9 is a graph showing the diffraction efficiency f1, light intensitydistribution f2, and output light quantity f3 with respect to incidenceangles when the light guide plate of the flat backlight of FIG. 7 has adot-patterned or rugged bottom surface with a grating interval of 440 nmand a grating depth of 0.25 nm. As shown in FIG. 9, the diffractionefficiency f1 remarkably decreases after the incidence angle reaches50°, while the light intensity distribution f2 remarkably increasesafter the incidence angle reaches 50°. The output light quantity f3 oflight output from a light guide plate corresponds to the product of thevalues of the two graphs f1 and f2, and has a maximum between 55° and70°.

That is, in the backlight with no light path changing units, thediffraction efficiency of light is the lowest at the incidence angle of90° where the light intensity is the highest, such that the output lightquantity is generally lowered. However, the flat backlights having alight path changing unit, according to the first through sixthembodiments of the present invention, can reduce the loss of lightintensity and increase the diffraction efficiency.

FIG. 10A shows the pattern of a planar hologram 45 a formed on a lightguide plate 43 a in a backlight with no light path changing units asdescribed in FIG. 7. Referring to FIG. 10A, the planar hologram 45 a hasa stripe pattern. The stripes constituting the planar hologram 45 a areconsecutively arranged on the entire bottom surface of the light guideplate 43 a.

In a flat backlight according to the present invention, when thediffraction efficiency of the planar hologram 45 a is uniform over thelight guide plate 43 a, the planar hologram 45 a becomes smaller as itapproaches the light source 41 a, and it becomes bigger as it becomesmore distant from the light source 41 a. Alternatively, the pattern ofthe planar hologram 45 a becomes sparser as it approaches the lightsource 41 a, and it becomes denser as it distances from the light source41 a.

If the light source 41 a is located on the lateral side of the lightguide plate 43 a, the light intensity becomes higher as it approachesthe light source 41 a, and it becomes lower as it distances from thelight source 41 a.

Accordingly, in order to emit uniform light from the entire surface ofthe light guide plate 43 a, it is preferable that planar holograms 45 awith low diffraction efficiency are arranged in the directionapproaching the light source 41 a and planar holograms 45 a with highdiffraction efficiency are arranged in the direction distancing from thelight source 41 a. If the planar hologram 45 a has uniform diffractionefficiency over the light guide plate 43 a, it is preferable that thepattern of the planar holograms 45 a becomes smaller as it approachesthe light source 41 a and becomes bigger as it distances from the lightsource 41 a.

The striped-patterned planar hologram 45 a can partially be formed onthe light guide plate 43 a. Preferably, the stripes of the displayhologram 45 a are periodically arranged. If the stripes of the displayhologram 45 a are not periodically arranged, light is not uniformlyemitted from the planar hologram 45 a, thus causing a degradation in thelight efficiency of the entire backlight.

FIG. 10B shows the pattern of a planar hologram 65 a formed on a lightguide plate 63 a in the flat backlights according to the first throughfourth and sixth embodiments of the present invention. Referring to FIG.10B, the pattern of the planar hologram 45 a becomes sparser in thedirections toward the light source 61 and the light path changing unit69, and becomes denser in the direction toward the center of the lightguide plate 63 a.

As described above, the light intensity of the planar hologram 65 a,near the light source 61 and the light path changing unit 69, isrelatively higher than that of the planar hologram 65 a at the center ofthe light guide plate 63 a. Accordingly, in order to uniformlydistribute the light intensity of light emitted from the light guideplate 63 a over the entire surface of the light guide plate 63 a, thediffraction efficiency of the planar hologram 65 a is reduced in thedirections toward the light source 61 and the light path changing unit69, and it is increased in the direction toward the center of the lightguide plate 63 a.

Here, the pattern of the planar hologram 65 a of FIG. 10B can be appliedto the flat backlights according to the first through fourth embodimentsand the sixth embodiment of the present invention. The grating intervaland the grating depth, depending on the incidence angle providing themaximum diffraction efficiency, control the diffraction efficiencywithin a predetermined range. When a pattern having uniform diffractionefficiency is formed, a planar hologram with a small pattern is formednear the light source 61 and the light path changing unit 69, and aplanar hologram with a large pattern is formed far from the light source61 and the light path changing unit 69.

FIG. 11A shows a rectangle-patterned planar hologram 45 b formed on alight guide plate 43 b in the flat backlight with no light path changingunits of FIG. 7. Referring to FIG. 11A, a planar holograms 45 b formedon a light guide plate 43 b is patterned with two types of rectangleshaving different grating intervals. As described above, the gratinginterval varies according to the wavelength of light.

The patterned rectangles of the planar hologram 45 b become smaller inthe direction toward the light source 41 b, and they become larger inthe direction distancing from the light source 41 b.

When the diffracting direction varies according to the wavelength oflight, the light intensity distribution depending on the angle variesaccording to the wavelength of light. This may cause a degradation inthe quality of image. Accordingly, the impression of a color shown on aliquid crystal panel may vary. If the planar hologram 45 b having theabove-described grating interval is formed on the light guide plate 43 bto make red light have the highest diffraction efficiency, thereproduced light may provide a stronger impression of red, which lowersthe image quality.

Accordingly, the planar hologram 45 b having at least two types ofgrating intervals can be provided in order to balance the impression ofa color.

As described above referring to FIG. 11A, the planar hologram 45 b canbe patterned in one type of rectangles such that the red light isemitted forward, and in the other type of rectangles such that the bluelight is emitted forward.

The grating interval satisfying Equation 2 is about 528 nm for red light(λ=800 nm), and about 474 nm for blue light (λ=405 nm). Thus, the use ofthe planar hologram 45 b patterned with rectangles having the two typesof grating intervals, i.e., 474 nm and 274 nm, can uniformly maintainthe color impression of emitted light.

The pattern for the planar hologram 65 b of FIG. 11B is obtained byapplying the pattern for the planar hologram 45 b of FIG. 11A, and canbe applied to the first through fourth and sixth embodiments of thepresent invention.

In FIG. 11B, the planar hologram 65 b is patterned with rectangleshaving two types of grating intervals based on the wavelengths of lightas in FIG. 11A. However, the planar hologram 65 b is patterned so thatthe diffraction efficiency becomes lower in the directions from thecenter of the light guide plate 63 b to both the light source 61 and thelight path changing unit 69. Alternatively, when the diffractionefficiency is uniform over the entire surface of the light guide plate63 b, the planar hologram 65 b is patterned with smaller rectangles inthe directions from the center of the light guide plate 63 b to both thelight source 61 and the light path changing unit 69. Therefore, thecolor impression of output light can be uniformly maintained, and thelight intensity of output light can be uniformly distributed over theentire surface of the light guide plate 63.

Alternatively, the planar hologram 65 b can be patterned such that twotypes of rectangles have different diffraction efficiency depending onthe wavelength of light by having different grating depths.

FIG. 12A shows the pattern of a planar hologram 45 c having twodifferent periodical-shaped grating intervals formed on a light guideplate 43 c in a backlight with no light path changing units according tothe present invention. Referring to FIG. 12A, the planar hologram 45 cis formed on the entire surface of the light guide plate 43 c, such thatthe diffraction efficiency of light increases. This increases theprobability that light is emitted from the light guide plate 43 c. Inaddition, as described above, light having two different wavelengths canbe output at an angle of 80° or more to the plane of the light guideplate 43 c, such that the color impression of light reproduced on aliquid crystal panel is equalized, and that a uniform light intensitydistribution is obtained. This leads to the improvement of imagequality.

In a flat backlight having no light path changing units according to thepresent invention, the planar hologram 45 included on the light guideplate 43 can be patterned with periodical stripes having no less thantwo different grating intervals. Each of the grating intervals can havea width satisfying Equation 4, depending on the wavelength of light.

FIG. 12B shows the pattern of a planar hologram 65 c formed on a lightguide plate 63 c in the backlights according to the first through fourthand sixth embodiments of the present invention. Referring to FIG. 12B,the diffraction efficiency of a planar hologram 65 c becomes lower inthe directions toward the light source 61 and the light path changingunit 69. Alternatively, when the diffraction efficiency is uniform overthe surface of the light guide plate 63 c, the pattern of the planarhologram 65 c becomes smaller in the directions toward the light source61 and the light path-changing unit 69. Each of the grating intervals isformed depending on the wavelength of light as shown in FIG. 12A, whilethe pattern of the planar hologram 65 c has different densities over thesurface of the light guide plate 63 c. However, this configuration isessentially made to evenly distribute light intensity of output lightover the entire surface of the light guide plate 63 c, similar to theprinciple of patterning of the planar holograms used in the flatbacklight having no light path changing units according to the presentinvention.

Referring to FIG. 13, in order to form a planar hologram 45 on a lightguide plate 43, first, the light guide plate 43 is coated with aphotosensitive material 50. Two incident light beams 31 a and 31 boutput from an identical light source are incident at angles Θ₁ and Θ₂upon the light guide plate 43 coated with the photoresist 50, andinterfere with each other. The interference between the two incidentlight beams 31 a and 31 b produces an interference pattern on thephotoresist 50 of the light guide plate 43.

The incident light beams can be formed of a combination of a convergentlight beam and a parallel light beam or a combination of the convergentlight beam and a diffused light beam. The planar hologram 45, which isengraved according to the type of incident light beams, has a gratinginterval P according to Equation 5:P=λ/(sin Θ₁+sin Θ₂)  (5)wherein the grating interval P is set to be 2 μm or less.

The planar hologram 45 can be obtained by developing or chemicallytreating the interference pattern using a standard process for thephotoresist 50. The planar hologram 45 can be mass-produced throughreproduction by stamping. Alternatively, the planar hologram 45 can beformed by exposing the light guide plate 43 to light using a mask.

In FIG. 13, if a volume hologram material such as dichromated gelatin orsilver halide is used as the photoresist, a volume hologram may beobtained. However, the present invention refers to a planar hologram. Aplanar hologram is easily mass-produced using the above-describedmethod, and provides a high environmental reliability in duplicates.

A planar hologram 45 having a uniform grating interval according to anembodiment of the present invention is manufactured by the interferencebetween two parallel light beams. In addition, a planar hologram 45having a gradually varying grating internal can be manufactured bychanging the angle made by two light beams using a convergent light beamor a parallel light beam.

FIG. 14 is a graph showing the diffraction efficiency of P-polarizedlight and S-polarized light according to the depths of the grating of aplanar hologram when blue light with a 460 nm wavelength and red lightwith a 620 nm wavelength are incident upon a planar hologram 45 with agrating interval of 440 nm. Here, the P-polarized light is indicated bytransverse magnetic light (TM) and the S-polarized light is indicated bytransverse electric light (TE). Accordingly, P-polarized light with a460 nm wavelength is indicated by 460TM, S-polarized light with a 460 nmwavelength is indicated by 460TE, P-polarized light with a 620 nmwavelength is indicated by 620TM, and S-polarized light with a 620 nmwavelength is indicated by 620TE.

As shown in FIG. 14, the diffraction efficiency of light 620TE has amaximum of about 20% at 0.3 μm and 0.6 μm grating depths, and thediffraction efficiency of light 620TM has a maximum of about 50% at a0.7 μm grating depth. It is known that each of the light 460TE and thelight 460TM has a diffraction efficiency of less than 10%.

When both a 620 nm light beam and a 460 nm light beam are incident uponthe flat hologram 45 with a 440 nm grating interval and a grating depthof about 0.25 μmm, only polarized light TE is usually output becausepolarized light TE has a significantly high diffraction efficiency.Accordingly, specific polarized light can be strengthened.

A lighting system for emitting only specific polarized light with highefficiency increases the light intensity of output light by reducing alight loss. Hence, such a lighting system can be useful in a displaydevice using polarization.

The effects of such a lighting system according to the present inventioncan be known from FIGS. 15A and 15B. FIG. 15A shows the light intensitydistribution of light transmitted by the light guide plate 43 in thebacklight with no light path changing unit according to the presentinvention. A light intensity of no less than 4,000 cd is distributedalong the horizontal axis 0°-180°. The light intensity at the center ofthe light guide plate 43 is about 4,600 cd, and the other areas of thelight guide plate 43 have light intensity of no more than 1,600 cd.

FIG. 15B shows the light intensity distribution of final lighttransmitted by a diffusion plate 49 through the light guide plate 43 inthe backlight of FIG. 7. Referring to FIG. 15B, the asymmetricalstructure of FIG. 15A is changed into a nearly symmetrical structure.The light intensity moves toward the vertical axis 90°-270° to form acircle around the center of the light guide plate 43. This leads touniform distribution of light. Here, the light intensity at the centerof the light guide plate 43 is about 2,400 cd.

It can be confirmed from FIGS. 15A and 15B that light can be verticallyoutput by using a light guide plate and a diffusion plate without usinga prismatic plate.

In a backlight according to preferred embodiments of the presentinvention, a planar hologram formed on a light guide plate enables lightto be output at an angle nearly perpendicular to the light guide plate.That is, as the light efficiency can be increased just by using a planarhologram, a backlight according to the present invention can bemanufactured in a simple structure requiring no prismatic plates.

Flat backlights according to preferred embodiments of the presentinvention provide a maximum diffraction efficiency by making the most ofthe incident light with a high light intensity. In addition, the flatbacklights according to the present invention increase luminance andreduce light loss by uniformly distributing light intensity over theentire surface of a light guide plate. Therefore, backlights of goodperformances are provided.

As described above, a flat backlight according to the present inventioncan reduce a light loss and increase the light intensity of output lightby adopting a light path changing unit to maximally diffract incidentlight having a maximum light intensity.

Also, a flat backlight according to the present invention can emit lightnearly vertically by periodically patterning a planar hologram, suchthat it can be manufactured in a simple structure providing a highluminous efficiency.

In addition, a flat backlight according to the present invention canincrease the diffraction efficiency of specific polarized light bycontrolling the depth of grating formed on a planar hologram.

While this invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

1. A backlight for a flat display device comprising: a visible lightsource; a light guide plate installed at one side of the light source,in which light from the light source travels while being totallyreflected; a planar hologram formed on at least one surface of the lightguide plate, the planar hologram having a pattern of a predeterminedgrating interval and a predetermined grating depth in order to diffractlight at a predetermined angle toward the light guide plate; and a lightpath changing unit installed at one side of the light guide plate, thelight path changing unit changing the path of light traveling in thelight guide plate to make light incident upon the planar hologram at anangle near the angle providing the maximum diffraction efficiency. 2.The backlight for a flat display device of claim 1, further comprising areflecting plate installed on the rear side of the light guide plate,the reflecting plate reflecting light diffracted by the planar hologramand sending the diffracted light back to the light guide plate.
 3. Thebacklight for a flat display device of claim 1 or 2, wherein the lightpath changing unit is a reflective mirror that is located opposite tothe light source and inclined at a predetermined angle.
 4. The backlightfor a flat display device of claim 1 or 2, wherein the light pathchanging unit is a reflective surface of the light guide plate, thereflective surface being located opposite to the light source andinclined at a predetermined angle.
 5. The backlight for a flat displaydevice of claim 1 or 2, wherein the light path changing unit is arefracting element installed between the light source and the lightguide plate or installed opposite to the side of the light guide platewhere the light source is installed.
 6. The backlight for a flat displaydevice of claim 5, wherein the refracting element is a refractive lens.7. The backlight for a flat display device of claim 5, wherein therefracting element is a refractive grating.
 8. The backlight for a flatdisplay device of claim 1 or 2, wherein the diffraction efficiency ofthe planar hologram becomes lower toward the light path changing unit.9. The backlight for a flat display device of claim 1 or 2, wherein thepattern of the planar hologram becomes smaller toward the light pathchanging unit.
 10. The backlight for a flat display device of claim 7,wherein the grating interval of the planar hologram is 2 gm or less. 11.The backlight for a flat display device of claim 9, wherein the gratinginterval of the planar hologram is no greater than 2 μm.
 12. Thebacklight for a flat display device of claim 8, wherein the gratinginterval of the planar hologram is composed of at least two types ofgrating intervals depending on the wavelength of light.
 13. Thebacklight for a flat display device of claim 9, wherein the gratinginterval of the planar hologram is composed of at least two types ofgrating intervals depending on the wavelength of light.
 14. Thebacklight for a flat display device of claim 1 or 2, wherein the gratingdepth of the planar hologram is set so that the diffraction efficiencyratio of two polarized light beams that meet each other at a right angleis no less than 1.5.
 15. The backlight for a flat display device ofclaim 1 or 2, further comprising a diffusion plate installed on theentire surface of the light guide plate to diffuse light emitted fromthe light guide plate.
 16. The backlight for a flat display device ofclaim 1, wherein said visible light source is a white light source. 17.The backlight for a flat display device of claim 1, wherein saidbacklight is free of any prismatic plates.
 18. The backlight for a flatdisplay device of claim 8, wherein the grating interval of the planarhologram is 2 gm or less.